First off, you be needing to select a character race, of course! Let's go through all the options we be havin':

Alliance side:
- Humans: Arr! The first real choice for a real buccaneer! Be it a matey or a lass, this shall be a good choice!
- Night Elves: Yo-ho, those be a fine choice too! Livin' in a island like theirs is a jolly good reason to set sail around Azeroth!
- Dwarves: Grog! Grog everywhere! Yo-ho-ho! Fine mateys, dwarves be!
- Gnomes: Those sneaky little scallywags! Fine they be, for the plunderin' of booty!
- Draenei: Those sure have nice wenches, so it's always nice to be havin' them around your furner.
- Worgen: Those be fine like them humans, except for the little more aggressive side, not good for the lily-livered.

Horde side:
- Orcs: Them green-skinned be very strong-hearted, they be making great swashbucklers!
- Trolls: "Stay away from da voodoo", as they be sayin'! They be great mateys, when they be not turnin' our partners in frogs or somethin'...
- Tauren: Grand big mateys, bloody perfect for the sweet trade!
- Forsaken: I don't like how they be smelling, but I do like their attitude!
- Blood Elves: Not sure if their males be ladies or mateys, but their female sure be fine to have aboard!
- Goblins: Squiffy little buggers, those be wantin' all the booty in the world! Be one, or be afraid of one!

Note on the character class
If you are creating a new character just to roleplay a pirate, the Warrior class is the best you can get, for the variety of weapons and equipment it can wear.

Now I think you be happy with your choice, right matey? Now you be needing some gear! This world of Azeroth brings lots of booty to gear you like a sprog or like a corsair! Let's be havin' a look at our choices:

- Head:
-- Hats:Admiral's Hat: This one sure is pretty! Fine like a privateer, you be getting this from a tailor who may know how to be crafting this fine piece of headgear. Such pattern can be obtained in the beaches of Stranglethorn Vale, by that Cowardly Crosby.

First Mate hat: YO-HO-HO, so you want to look like a real pirate, jolly roger and everything? You shall be lookin' for this. It's known that very few members of the Bloodsail have the honors to carry one of this fine hats, so they can be bloody stinkin' rare. G'luck gettin' one of these, but if you do, enjoy!

--Eyepatches:Malefactor's Eyepatch: This mail-crafted eyepatch is obtained from the quest Showdown, in the far reaches of the Blade's Edge Mountains, in Outland.

Buccaneer's Boots: Comfortable! Those fine boots can be found randomly around the world, because some squiffy lost them at areas of 17-22 level range. Or you can be usin' your booty to get one of those from the Auction House.

- Weapon

Cutlass: Good ol' trusty Cutlass can be found in many weapon shops all around Azeroth, get yours while they're hot! Or any other weapon that looks like it.

Speedsteel Rapier: This great swashbucklin' device be found randomly at chests or areas of level range 40-43, or purchased from the Auction House. Or you can be finding some similar weapons around!

Solid Blunderbuss: Yarr! You cannot be leavin' your shit without one of these! A good buccaneer is that one prepared to blast a scurvy dog head off with his trusty blunderbuss. This is sold by many vendors all around Azeroth!

Yo-ho-ho! So you want to become a Bloodsail Buccaneer, do you? For this you must be strong! Get rid of that scurvy dog named Revilgaz and the rest of his bloody damned town of Booty Bay, and in no-time the Bloodsail Buccaneers shall give you all of their equipment, to look just like them! But be careful, as the goblins of the Steamwheedle Cartel will be hating you for this!
A great guide on getting this reputation would be this one, hosted at warcraftpets.com.

A pirate needs company all times, be it of a lass or of a friendly animal! The best choice for a minipet, of course, shall be the parrots. There be many varieties, which you may find both as a item from a vendor, or from a very, very rare drop from those damned Bloodsail Pirates.

Oh, so you just want to be a pirate for a while? We have a solution for ya as well! The magic of the World of Warcraft allows you to be transformed in a pirate in many ways:

Savory Deviate Delight: This tasty meal is cooked from a fish that can be... fished, right outside the Wailing Caverns in the Northern Barrens. But be careful, eatin' this can either make you a fine human buccaneer or a son of a biscuit eater ninja, and you shall walk the plank!

Hallowed Wand - Pirate: This magical device transforms your target into a pirate! So if you want to transform yourself, you'll be needin' a friend with it! This item is obtained from the Hallow's End event in Azeroth every year. Unlike the Savory Deviate Delight, it transforms you into a pirate of a random race instead of only human.

- This is a very generic roleplay tip but: be creative! Make a cool background for your character, tell everyone your plunderin' and swashbucklin' stories at a pub while drunk, and have fun with it. Do what you want because a pirate is free!

Those swashbuckler's shirts are nice, the best is Captain Sanders' Shirt.
You get his by killing murlocs on the northern shore of Westfall until one of them drops a treasure map:
(wowhead) item=1357
This leads you on a scavenger hunt all over Westfall in a 4-quest chain.
Eventually you'll get to the locked chest on a tiny island.
And inside you'll find this famous shirt.
(wowhead) item=3342

Note that this shirt isn't that great on female models (stick with the Swashbuckler!).